An ex-girlfriend of Boston Marathon bomber Tamerlan Tsarnaev said the man she started dating in 2006 transformed from a pot-smoking party boy into a jihad-spouting Islamic terrorist during their three-year relationship.

Nadine Ascencao, 25, of Cambridge, Mass., said she was 17 when she fell for the rugged Tsarnaev — who was 26 when he was killed by cops days after setting off two bombs at the Boston Marathon.

When she liked him, he was an aspiring boxer with a taste for flashy clothes.

But as his career in the ring crashed, he grew increasingly enraged and immersed himself in violent, anti-American jihadi videos and Web sites.

“One minute, he’s this funny, normal guy who liked boxing and having fun. The next, he is praying four times a day, watching Islamic videos and talking insane nonsense,” she said.

“He became extremely religious and tried to brainwash me to follow Islam. Tamerlan said I couldn’t be with him unless I became a Muslim. He wanted me to hate America like he did,” Ascencao told Britain’s The Sun newspaper.

“Tamerlan had taken my virginity and said he loved me because I was pure and hadn’t been with any other guys. I was in love and scared he’d leave me if I didn’t do what he said. Looking back, I had a lucky escape.”

Ascencao said she was so smitten, she actually considered converting after shacking up with Tsarnaev in a shabby apartment on Norfolk Street in a rundown section of Cambridge.

“I went to his mosque a couple of times and even looked into converting to make him happy. I thought, ‘This is crazy’ — but I still did it for him,” she said.

But his crazy ravings and violent nature soon scared her off, and she never followed through.

“Tamerlan told me I should only talk to Muslim girls, not other ‘slutty’ girls.’ He once ripped a pair of my jeans and hit me in the face with them,” Ascencao said, adding that he insisted she wear the traditional Muslim hijab.

The muscle-bound brute finally lost it when she went to a pool party with pals wearing a tank top and skimpy cutoffs.

When Ascencao heard the news about the bombings — which killed three and injured more than 200 — she wasn’t upset he was killed.

“When they said Tamerlan was dead, I didn’t cry,” Ascencao said, though she was shocked that his kid brother Dzhokhar was part of the deadly plot, which ended in a wild chase and shoot-out.

“I was more shocked Dzhokhar was involved,” she said. “He was a nice kid.”

The younger brother’s defense team got a major boost yesterday with the addition of noted attorney Judy Clarke. The death penalty lawyer has represented a variety of famed rogues from Ted Kaczynski to Susan Smith to Jared Loughner.